the main aluminium alloys used for MTB parts have different qualities so to some extent it would depend what you want from the component and how you are planning on manipulating it which series would be best.

aluminium alloys are wee bit different the two high strength ones most folks know about are your 2000 series and 7000 series
does it not have a marking on its length aluminium usually has a black dot etch describing temper and chemical makeup

2000 series are pretty hard to weld 7-- series not hard to weld till you hit the 7075 alloys and then thats just using fusion welding there are other ways of welding upper end 7000 series alloys and 2000 series alloys without fusion welding but they are ok strength wise

6061 is like the does lots of stuff aluminium alloy

but it depends like steel what he heat reatment and alloying content are

2000 sounds low on the scale which considering it follows 1000 series its quite a shocker you can make ot quite strong

5000 series is a good un for toughness but in pretty thick sections

if its a CDT or craft tech class you probably know it as he30 in a T6 temper which is a 6000 series alloy if its wrought alloy its pretty hard if its extruded or bar extruded stock it will be a soft aluminium sometimes in what is called O spec but maybe T4 ITS HARD TO TELL UNLESS ITS ON A BENCH IN FRONT OF YOU

And what would be regarded as a fairly high quality aluminium (ie 6061 etc etc) for use on a mtb part?

As others have probaly said (though i haven't actually read as jsut bak from pub) it depends upon what u want to use it for. 6061 is more of a US specific compisition and the uk equiv is 6082 more commonly known as HE30